Computer keyboard failures can be attributed to electrical defects or mechanical defects. A repair facility currently has 25 failed keyboards, 6 of which have electrical defects and 19 of which have mechanical defects.
a. How many ways are there to randomly select 5 of these key boards for a thorough inspection (without regard to order)?
b. In how many ways can a sample of 5 keyboards be selected so that exactly two have an electrical defect?
c. If a sample of 5 keyboards is randomly selected, what is the probability that at least 4 of these will have a mechanical defect?